The field of active flexible microwave packaging includes numerous attempts to optimize a package for popping corn in a microwave oven. Many of the early United States patents in the field of packaging for microwaveable food products, (U.S. Pat. No. 4,219,573 to Borek, U.S. Pat. Nos. 4,553,010 and 4,678,882 to Bohrer, and U.S. Pat. Nos. 4,735,513 and 4,878,765 to Watkins) describe microwaveable popcorn bags incorporating a susceptor patch.
In each case, the bag is loaded with a portion of popping corn and oil, and that portion is positioned over the susceptor patch. The size of the patch is limited by the desire to eliminate scorching and overheating of the bag which occurs when the patch is of a size greater than the food load positioned adjacent to the susceptor.
Another active microwave package (not designed specially for popcorn) is described in U.S. Pat. No. 4,890,439 to Smart. The Smart patent suggests the use of a package having multiple susceptor patches as well as the requirement to eliminate the susceptor from the seal areas of the package in order to prevent the bag from unsealing during microwave heating. The Smart patent also seeks to prevent the seal itself from disintegrating.
Canadian patent application 2,033,775 to Mason et al describes a microwave cooking bag defining an enclosed cooking space substantially surrounded by a susceptor material. More particularly, Mason et al. teach excluding susceptor material from the fin seal area of a bag and further teach excluding susceptor material in an area in the proximity of the open end of the bag. The reference also suggests varying the area of the susceptor surface for different heating performance.
The intensity of the microwave field within a microwave oven is a significant problem for flexible packaging having microwave interactive elements. These interactive elements heat quickly and significantly when irradiated with microwave energy, especially in susceptor areas not in contact with a food product, since the food product acts as a heat sink and a microwave shield. The resulting intense heating has, in the prior art, limited positioning of the susceptor to areas that approximate the size of the food load. This has inhibited commercialization of a general purpose active microwave bag for retail consumer applications.
Scorching, burning and unwanted sealing can occur when two or more microwave active layers come in close proximity to one another. For instance, wherever two surfaces of a heat-sealable susceptor laminate are heat-sealed to form a bag, the seal areas, when exposed to microwave energy get extremely hot, perhaps even hot enough to cause damage to the package. (In fact numerous cases have been documented in which conventional susceptor-containing packages have caught fire in consumer microwave ovens.)
The fused susceptor described in application Ser. No. 08/187,446 U.S. Pat. No. 5,412,187 entitled FUSED MICROWAVE CONDUCTIVE STRUCTURE and incorporated herein by reference provides a system of increasing the susceptor area of a package without increasing the risk of burning and scorching of the package in unloaded or non-shielded areas.